MONDAY, Feb. 1, 2016 (HealthDay News) -- Exposure to high levels of air pollution in pregnancy may increase the risk of having a preterm baby, new research suggests.

For the study, researchers examined nearly 225,000 births of single babies in Ohio between 2007 and 2010. More than 19,000 of them were preterm deliveries -- before 37 weeks of pregnancy.

Exposure to high levels of small particle air pollution during pregnancy was associated with a 19 percent increased risk of preterm birth. The risk was greatest when high levels of exposure occurred during the third trimester, the study found.

"Although the risk increase is modest, the potential impact is robust, as all pregnant women are potentially at risk," study author Dr. Emily DeFranco, a physician-researcher at the Center for Prevention of Preterm Birth of Cincinnati Children's Hospital Medical Center, said in a medical center news release.

The type of air pollution looked at in the study is composed of small particles from car exhaust or burning wood, coal and other fossil fuels. According to the U.S. Environmental Protection Agency, this type of air pollution can be inhaled deep into the lungs.

Preterm birth rates were highest among women 40 and older, black women, those with no prenatal care or with lower education level, and those exposed to levels of small particle air pollution above the EPA standard, according to the researchers.

The findings were published online recently in the journal Environmental Health.

The report doesn't prove that exposure to air pollution causes premature births, but the researchers believe the association is significant.

"We estimate that decreasing the amount of particulate matter in the air below the EPA's standard threshold could decrease preterm birth in women exposed to high levels of small particulates by about 17 percent, which corresponds to a 2.22 percent decrease in the preterm birth rate in the population as a whole," DeFranco said.

In a previous study, she found that exposure to high levels of particulate air pollution in the third trimester of pregnancy was associated with a 42 percent higher risk of stillbirth.

Last year, the American Lung Association listed two areas in Ohio -- Cincinnati-Wilmington-Maysville and Cleveland-Akron-Canton -- among the 10 worst regions in the United States for year-round particle pollution.